This invention relates to a flow rate detection device for measuring a flow rate of a dielectric fluid.
As a flow meter for detecting a flow rate of a dielectric fluid such as a fuel or the like, a vane type flow meter such as that shown in FIG. 1 is used. That is, in FIG. 1 (1) is a flow path of a fluid, (2) a vane wheel, and (3) is a container for accommodating that vane wheel therein. When the fluid flowing through the flow path (1) strikes against the vane wheel (2), the vane wheel (2) is rotated at a speed identical to a flow speed of the fluid. Assuming that the flow speed of the fluid is designated by V in m/sec, the number N, in r/sec of rotation per unit time, (which is simply called hereinafter a rotational speed), of the vane wheel (2) is given by the following expression: EQU N=K.sub.n V (1)
On the other hand, assuming that a flow rate of the fluid is designated by Q.sub.v in m.sup.3 /sec and a sectional area of the flow path (1) is designated by S in m.sup.3, the flow speed V in m/sec of the fluid is given by the following expression: EQU V=(Q.sub.v /S) (2)
From the expressions (1) and (2) the relationship between the flow rate Q.sub.v in m.sup.3 /sec of the fluid and the rotational speed N in r/sec of the vane wheel (2) is given by the following expression: EQU N=K.sub.n (Q.sub.v /S) (3)
Since both the sectional area S in m.sup.2 of the flow path (1) and K.sub.n in the expression (2) are constants, the rotational speed N in r/sec of the vane wheel (2) is proportional to the flow rate Q.sub.v in m.sup.3 /sec of the fluid. Therefore, if the rotational speed N in r/sec is measured, the flow rate of the fluid flowing through the flow path (1) can be measured.
Since conventional vane flow meters measure the rotational speed N in r/sec of the vane wheel (2) as described above, the flow rate of fluid can be accurately measured as shown in the expression (3) provided that a flow of the fluid flowing through the flow path (1) is in the steady state (the flow rate Q.sub.v in m.sup.3 /sec being substantially constant). However, where the flow rate of the fluid has suddenly changed (suddenly increased or suddenly decreased), there is a time delay until the vane wheel (2) has a speed identical to the flow speed v in m/sec of the fluid because the vane wheel (2) has an inertia. The inertia of the vane wheel (2) can be reduced by properly selecting the shape and material of the vane wheel (2) and devising it so as to be small-sized and light-weight. However, since the strength of the structure can not be disregarded, a reduction in inertia of the vane wheel (2) has a limit. Accordingly, in the case where the flow of the fluid is suddenly varied with respect to time, for example, in the case of an intermittent stream, it is possible to comparatively accurately measure the mean flow rate but it is impossible to accurately measure an instantaneous flow rate because a time delay occurs until the vane wheel (2) speed coincides with the flow speed of the fluid as described above.